1. Field of the Invention
The present invention relates to a system for reducing crosstalk in a backplane. More particularly, the present invention reverses pin connections of traces extending between connectors on a backplane to reduce crosstalk.
2. Background of the Related Art
Electronic systems are often assembled from multiple printed circuit boards (PCB). Each circuit board, which is sometimes called a “daughter card,” contains electronic components. The daughter card includes one or more electrical connectors that allow the circuits on the daughter card to be connected to other circuits in the system.
Often, electrical signals are routed from one daughter card to another through a backplane. A backplane is a printed circuit board with conducting paths, called “signal traces” or just “traces,” such as a wire. Connectors are mounted on the backplane to make connection to the traces. When the connectors on the daughter cards are mated with the connectors on the backplane, signal paths are established between the daughter cards. The portion of a printed circuit board that allows a trace to be connected to a contact element in a connector or other electronic element is called a “signal launch” or “launch.”
As electronic systems have become smaller, faster and more complex, this has generally required that backplanes provide more signal traces without increasing in size, or while actually decreasing in size. As a result, electrical noise becomes more problematic. Electrical noise is usually considered any undesirable electrical energy in an electronic system, including but not limited to, reflections, electromagnetic interference, mode conversions and unwanted coupling, such as crosstalk.
The trend for smaller, faster and more complex electronic systems has also required connectors to carry more and faster data signals in a smaller space without degrading the electrical characteristics of the signal. Connectors can be made to carry more signals in less space by placing signal conductors in a connector closer together. A major difficulty with placing signal conductors closer together is that electrical noise between the signal conductors increases as the distance between signal conductors decreases and as the speed of the signals increases. In addition, as frequency content increases, there is a greater possibility of energy loss. Energy loss may be attributed to impedance discontinuities, mode conversion, leakage from imperfect shielding, or undesired coupling to other conductors such as crosstalk.
Differential signals are signals represented by a pair of conducting paths called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducing paths of a differential signal pair are arranged to run near each other. If any other source of electrical noise is electromagnetically coupled to the differential signal pair, the effect on each conducting path of the pair should be similar. Because the signal on the differential signal pair is treated as the difference between the voltages on the two conducting paths, a common noise voltage that is coupled to both conducting paths in the differential signal pair does not affect the signal. This renders a differential signal pair less sensitive to crosstalk noise, as compared with a single-ended signal path. At high speeds, the available electrical connector may encounter crosstalk, impedance and attenuation mismatch characteristics. And the signal transmission characteristics degrade.
Crosstalk generally refers to electromagnetic interference that comes from an adjacent wire. Current that flows down one of the traces, referred to as the aggressor line, couples into the adjacent trace, called the victim line, and creates two different noise signals in the victim line. One of the noise signals, called “forward crosstalk” or “far end crosstalk,” flows in the victim line in the same direction as the current flowing in the aggressor line. The other noise signal, called “backward crosstalk,” flows in the victim line in the opposite or backward direction as the current flowing in the aggressor line.
Referring to FIG. 1(a), a standard backplane connection system 100 is shown having a first connector 110 and a second connector 120 with various traces. The first connector 110 has inputs receiving traces 130, 132. Traces 134, 136 are provided to connect the first connector 110 and the second connector 120. And, the second connector 120 has output traces 138, 140.
The impact of crosstalk on the system 100 is exemplified in FIGS. 1(b)-(i). In the embodiment of FIG. 1(a), the bottom traces 130, 134, 138 are the aggressor traces and the top traces 132, 136, 140 are the victim traces. Once the signal on the aggressor line pair 130 reaches the first connector 10, FIG. 1(b), noise in the form of crosstalk is created at the first connector 110, as shown in FIG. 1(c). The noise is generated inside the first connector 110, which has coupling between pairs. Noise is also generated in the footprint of the connector 110, which is the portion of the Printed Wiring Board (PWB) that the connector 110 is pressed into. The connector 110 has attachments to the board, such as surface-mount or compliant pin. In addition, the vias created in the board are cylinders through the thickness of the board. These via cylinders can also couple to each other and generate noise. This effect is directly related to the thickness of the board, whereby the thicker the board is, the more crosstalk that is generated. For a thick board, the crosstalk in the board can be greater than the crosstalk from the connector.
There is no noise on the second connector 120 at this point, FIG. 1(d), so that the noise at the first connector 110 is the total noise for the system, FIG. 1(e). The noise from the first connector 110 continues at the output of the first connector 110 on the victim trace 136.
Once the signal reaches the second connector 120, FIG. 1(f), there is the noise on the victim trace 136 from the first connector 110, FIG. 1(g), and the noise that is generated at the second connector 120, FIG. 1(h). Noise is generated in the second connector 120 the same as it is generated in the first connector 110, i.e., due to the coupling between pairs, the footprint attachments, and the via cylinder coupling. The noise on victim trace 136 and the noise on the second connector 120 get combined inside the second connector 20. Thus, the total noise in the system 100, FIG. 1(i), is about doubled that at each connector 110, FIGS. 1(g), (h).
While it is possible to minimize the deleterious coupling effects by adding space between the victim and aggressor lines, that comes at the disadvantage of losing density in the backplane. Thus, as ever more signals are populated onto the card, the problem with coupling becomes more probable and problematic.
In a backplane environment, the lines 130, 132 coming into the first connector 110 can be, for instance, from a line card of a router, or a processor card. The lines 138, 140 out from the second connector 120 can be to a fabric card or a switch card.